This invention relates to wireless communication. More particularly, this invention relates to use of polarized communication signals.
Prior art systems accept the long-recognized constraint imposed by Maxwell's equations that signals which are transmitted from point A to point B over a free space path that directly connects points A and B, and which differ only in their polarization modes, can comprise at most two independent channels. The reason for this constraint lies in the fact that the polarized transmission coefficients between points A and B form a matrix, T, of rank 2. The prior art, therefore, were always of the view that signals can be usefully transmitted from a point A to point B at most with two polarizations, and realizing thereby at most two independent channels of communication. This is demonstrated in the prior art system of FIG. 1, where a transmitter 10 has one dipole antenna 11 and another dipole antenna 12 and a receiver 20 has one dipole antenna 21 and another dipole antenna 22. Typically, dipole antennas 11 and 12 perpendicular to each other, and so are dipole antennas 21 and 22. The most efficient transfer of information from the transmitter to the receiver occurs when antennas 11 and 12 are in a plane that is perpendicular to the line connecting points A and B, antennas 21 and 22 are in a plane that is parallel to the plane of antennas 11 and 12, and antenna dipole 11 is also in a plane that contains antenna 21. Of course, any other spatial arrangement of antennas 11, 12, 21 and 22 may be used for communicating information from the transmitter to the receiver, except that the effectiveness of the communication is reduced (a greater portion of the transmitted signal energy cannot be recovered), and the processing burden on the receiver is increased (both antennas 21 and 22 detect a portion of the signal of antenna 11 and of antenna 12).
Whether a transmitter has a single antenna (polarized or not) or two polarized antennas (as in FIG. 1), it remains that multi-pathing presents a problem. Specifically, multiple paths can cause destructive interference in the received signal, and in indoor environments that presents a major problem because there are many reflective surfaces that cause multiple paths, and those reflective surfaces are nearby (which results in the multiple path signals having significant amplitudes).